The second page of the installer presents you with an option to “Add Python to environment variables”. This prepends both the installation and scripts directory to either the current-user or system
PATH, depending on whether an “all users” installation is selected. For an “all users” installation it also modifies the value of
PATHEXT to append “.PY” and “.PYW”.
After installation, if
PATH was updated, Explorer (the Windows graphical shell) should update its environment with the new value. The installer implements this by sending an “Environment” message to top-level windows, which causes Explorer to reload its environment. Instances of CMD or PowerShell have no windows (the console window is hosted by conhost.exe), so they don’t see the “Environment” window message. You have to start a new instance of CMD or PowerShell from Explorer to get the updated
PATH. After that, pip.exe in the “Scripts” directory should work. If not, you can fall back on using the py launcher plus the -m command-line option that executes a module as a script, e.g.
py -m pip install numpy --user.
You can learn more about the installation process and the launcher at the following page: https://docs.python.org/3/using/windows.html